The present invention relates to a holder for holding a double-side printed circuit panel for exposure to light.
More precisely, the invention relates to a system enabling two artworks to be held with great accuracy on either side of a panel on which a printed circuit is to be made, and then subjecting the two faces of the resulting assembly to light beams so as to expose both faces of the printed circuit panel through the artworks.
When making a double-sided printed circuit panel, i.e. when making conductor tracks on both faces of an insulating support, it is known that the starting assembly comprises an insulating support with respective conductive metal layers made on each of the faces of the insulating support, both of said conductive layers being coated in a photosensitive deposit. Thereafter, the photosensitive layers are subjected to light beams through respective artworks representing the shape of the conductor tracks in positive or negative form, and the photosensitive layer is thus transformed locally, making it possible subsequently to chemically etch away those portions of the photosensitive layer that have been exposed, or on the contrary, those portions of the photosensitive layer that have not been exposed, and then to chemically etch away the conductive layer through the resulting mask so as to leave only the conductor tracks in position.
Such a machine for exposing a panel to light is constituted by a mechanical assembly referred to as an exposure holder and by a light source suitable for delivering two light beams which expose both faces of the printed circuit panel. The present invention relates more particularly to the exposure holder, i.e. to the mechanical machine that makes exposure possible.
The exposure holder serves firstly to position the artworks very accurately on the two faces of the printed circuit panel, and secondly to apply the artworks directly against the face of the panel so as to avoid any diffusion effects during exposure to light.
European patent applications Nos. EP 618 505, EP 807 855, and EP 807 856 filed in the name of the applicant disclose exposure holders, each constituted by a structure carrying a bottom support for the bottom artwork, a top support for the top artwork, and means for positioning the panel between the two artworks. As a general rule, the top support is movable so as to come substantially into contact with the panel placed between the two frames. In addition, by establishing a vacuum between the artworks, close contact is obtained between them and the faces of the panel. The artwork supports are constituted firstly by respective rectangular frames and secondly by respective panes of glass or more generally of a rigid transparent material secured to the frames. The artwork is pressed with one face against the pane and is held thereto by a suction system.
The presence of the pane in the artwork support suffers from certain drawbacks. Firstly, the pane may have defects or scratches or dirt on a face thereof, which can disturb the definition of the surface to be exposed and thus degrade the accuracy with which the conductor tracks are made thereby.
In addition, because of its rigidity, a sheet of glass can present departures from planeness which run the risk of giving rise to defects in the application of the artwork against a wall of the printed circuit panel.
Also, the artwork support pane is relatively rigid, and when a vacuum is established between the top and bottom artwork-supporting panes, that can lead to a pane being broken, and thus to the machine being taken out of operation.
Finally, there is no guarantee that both faces of a pane are accurately parallel to each other. Variations in thickness, although very small, can also lead to the quality of the conductor tracks being degraded.
Nevertheless, it should be recalled that the thickness of the artwork itself is very small, so the artwork is very flexible and can therefore sag significantly if it is not supported by a face that is itself relatively rigid and constituted, in prior machines, by the artwork-supporting pane.
An object of the present invention is to provide a holder for exposing a printed circuit panel to light, the holder making it possible effectively to apply the artworks very accurately to both faces of the printed circuit panel while avoiding the drawbacks associated with the presence of a glass pane or of an analogous transparent support in the artwork-supports of the exposure holder.
To achieve this object, the invention provides a holder for exposing both faces of a panel to light in order to make a double-sided printed circuit or the like by using first and second flexible artworks disposed on either side of said panel, said holder comprising:
a first frame, having no transparent plane support member, for receiving the first flexible artwork;
a second frame, having no transparent plane support member, for receiving the second flexible artwork;
means for positioning and fixing the peripheries of said first and second artworks respectively to said first and second frames;
means for positioning and holding said panel between said first and second frames;
approach means for causing said frames carrying said artworks to move progressively towards the faces of said panel by traveling from a spaced-apart first position towards an exposure second position; and
means for establishing suction in the space between the frames fitted with their artworks and containing said panel, thereby causing said artworks to become pressed progressively against the two faces of said panel.
It will be understood that because the artwork is fixed around its entire periphery to the frame, which does not have any panes and more generally any transparent plane supports, it is possible, by moving the two frames towards each other, to press the artworks fixed to the frames progressively against the two faces of the panel. Preferably, the means for establishing suction are activated simultaneously with the means for moving the frames towards each other. This is made easier by synchronizing the movement of the frames, and thus of the artworks, towards each other with establishing suction in the space that extends between the two frames fitted with their artworks, thereby causing the relatively flexible artworks to be applied against the faces of the panel without risking the presence of bubbles of air between an artwork and a face of the panel.
In a preferred embodiment, the holder includes means for defining a pivot axis along the bottom edges of the frames, with said frames carrying their artworks being caused to move towards each other by pivoting about such pivot axis.
Also preferably, the suction means for generating suction are disposed close to the top edges of the frames, and thus to the top edges of the artworks.